Genetically obese fatty (fa/fa) male rats with abnormal oral glucose tolerance associated with initial hyperinsulinaemia as well as control lean (FA/FA) rats were investigated for the development of retinal microangiopathies. The animals were kept on a standard or sucrose supplemented diet. When tested at 60 weeks, the glucose intolerance of fa/fa rats was accompanied by an insulin response that was now either comparable to that of lean rats (standard diet) or close to nil (sucrose supplemented diet). At killing (68 weeks of age), retinal vasculature was examined by electron microscopy and morphological changes were quantitatively assessed by ultrastructural morphometry. A retinal microangiopathy was observed in all mutant animals which was more pronounced in the sucrose fed group, and which was characterized by: (1), an increase in focal thickenings and in nodules of the basement membrane adjacent to the perivascular glial cells: (2), a decrease in the number of pericyte nulei with concomitant signs of early degenerative cytoplasmic changes of pericytes; (3), an increase in the pinocytic activity of endothelial cells, indicative of presumptive changes in vascular permeability; (4), an increase in the number of intercellular endothelial junctions; (5), the presence of numerous stimulated platelets within capillaries. The fa/fa rat may thus be considered as a suitable model for studying the pathophysiology of ocular complications, in particular retinopathy accompanying non-insulin-dependent diabetes.